Anti-skid systems to control braking are well known, and the referenced U.S. patents describe various systems. Usually, wheel speed is sensed, and signals are derived therefrom, based on change of wheel speed, i.e. acceleration or deceleration, rate of change of deceleration, and the like. These signals, possibly combined with vehicle speed signals derived from extrapolation of previously derived vehicle speed signals, are then processed to provide control signals which control brake valves which, in turn, control the brakes of the respective wheels. The brakes are so controlled that the retardation or braking of the vehicle corresponds to that which the wheels can accept without skidding or slipping on the roadway on which they are running. Slip between the wheels and the roadway can be determined by sensing limit values of the control signals. The respective brakes of the wheels are then so controlled that, when the control signals reach or approach the limit values, indicating that the wheels tend to block, and hence may cause the vehicle to skid, braking pressure is relieved so that the wheels of the vehicle will run or turn just below the blocking limit.
If the roadway has highly asymmetrical friction with respect to the wheels, for example if the wheels on one side of the vehicle are on a dry or comparatively high-friction surface of the roadway, whereas the wheels on the other side of the vehicle are on a highly slippery surface, for example a patch of ice, a track of snow, or the like, application of braking pressure to the wheels of the vehicle may cause the vehicle to yaw. Substantial yawing torques may be applied. These yawing torques are torques which tend to rotate the vehicle about a vertical axis, since the wheels which are braked and which run on the high-friction surface area of the roadway tend to delay the vehicle substantially at that side, whereas the wheels which are on the slippery side of the roadway have no retardation effect on the vehicle. Such yawing torques are particularly dangerous when applied to trucks and, the longer the wheel base of the vehicle, the more pronounced will be the yawing if highly differential frictional conditions occur between the right and left sides of the vehicle as it is being braked.
It has previously been proposed to prevent the occurrence of high yawing torques by controlling the brakes at the side of the vehicle which have the high-friction contact with the road surface as a function of retardation or braking on the side having the low frictional value. Thus, it has been proposed not to drop the braking pressure at the side with high frictional value, but also not to increase it until an increase in braking pressure can be tolerated at the side having the low road friction value. This, however, reduces braking effectiveness at the side with the high road friction value, and the actual braking pressure is increased only in a predetermined value to the possible braking effects from the low road friction side. While this system is effective to reduce yawing, the braking length is increased, since braking at the high road friction side is lowered based on braking at the low road friction side.
An anti-skid system in which the high road friction side braking is controlled as a function of braking effort on the low road friction side is described in the referenced German Patent Disclosure Document DE-OS No. 28 30 809 to which U.S. Pat. No. 4,288,127, Leiber, Jonner and Goebels, corresponds.
Under extreme, almost "worst case" road conditions, particularly if the road friction differences between the tracks on which the wheels of respective sides of the vehicle are running are extreme, the braking distance may become excessively long. Also, remanent braking torques on the vehicle side having the low road friction value may interfere with proper operation; such difficulty may be caused by increased friction in bearings of the wheels, brakes which do not release exactly as commanded by the braking pressure, but have remanent adhesive friction and the like. The interaction of difficulties which can arise in braking under such conditions may interfere with proper build-up of braking pressure at the brakes of the side of the vehicle which has its wheels running on a high road friction track range of the roadway.